For example, vacuum pumps are used in semiconductor fabrication systems, and a motor as a drive source is provided thereto. As such a motor used in a vacuum pump, there is a motor having a structure in which a motor rotor is disposed directly on a pump main shaft. In the case of the motor having this structure, the motor rotor is exposed to a corrosive gas used in a semiconductor fabrication process. In addition, a sealing structure is realized to maintain a vacuum (low pressure) state in which a can is provided between a motor stator and the motor rotor (refer to Patent Document 1).
In addition, end rings are provided at both end portions of a rotor core of the motor rotor in the direction of a rotating axis thereof (refer to Patent Document 2). These end rings form an induction circuit together with the rotor core. However, since the motor rotor rotates at high speeds, in case there exists a rotation imbalance, large stress is applied to the pump main shaft, causing a deflection in the pump main shaft. In addition, there may be a case where the pump main shaft is caused to vibrate largely. As an approach to eliminate such a rotation imbalance, it is practiced that balance rings are attached to the pump main shaft so that the rotation of the pump main shaft is balanced by removing a little portion of the balance rings (refer to Patent Document 2).